Ports

On what kind of 'service' the server is providing and what definition you put on 'function'.

Write down a list of desired 'services' and look up the ports they require. There are plenty of sources of information on the Internet as to which ports are used for what purpose. Close all unused ports, i.e. the ones that are not in your list. Do the necessary services still work? That is to say, does the server still 'function'?

If so, you have the answer!